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Research ArticleMetal-Organic Framework MIL-101: Synthesis and
PhotocatalyticDegradation of Remazol Black B Dye
Pham Dinh Du ,1 Huynh Thi Minh Thanh,2,3 Thuy Chau To,1 Ho Sy
Thang,4
Mai Xuan Tinh,3 Tran Ngoc Tuyen,3 Tran Thai Hoa ,3 and Dinh
Quang Khieu 3
1Faculty of Natural Science, Thu Dau Mot University, 820000,
Vietnam2Department of Chemistry, Quy Nhon University, 59000,
Vietnam3University of Sciences, Hue University, 530000,
Vietnam4Dong Thap University, 870000, Vietnam
Correspondence should be addressed to Dinh Quang Khieu;
[email protected]
Received 20 January 2019; Revised 2 April 2019; Accepted 28
April 2019; Published 14 May 2019
Guest Editor: Soubantika Palchoudhury
Copyright © 2019 Pham Dinh Du et al. This is an open access
article distributed under the Creative Commons Attribution
License,which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly
cited.
In the present paper, the synthesis of metal-organic framework
MIL-101 and its application in the photocatalytic degradation
ofRemazol Black B (RBB) dye have been demonstrated. The obtained
samples were characterized by X-ray diffraction (XRD),transmission
electron microscope (TEM), X-ray photoelectron spectroscopy (XPS),
and nitrogen adsorption/desorptionisotherms at 77K. It was found
that MIL-101 synthesized under optimal conditions exhibited high
crystallinity and specificsurface area (3360m2·g-1). The obtained
MIL-101 possessed high stability in water for 14 days and several
solvents (benzene,ethanol, and water at boiling temperature). Its
catalytic activities were evaluated by measuring the degradation of
RBB in anaqueous solution under UV radiation. The findings show
that MIL-101 was a heterogeneous photocatalyst in the
degradationreaction of RBB. The mechanism of photocatalysis was
considered to be achieved by the electron transfer from
photoexcitedorganic ligands to metallic clusters in MIL-101. The
kinetics of photocatalytic degradation reaction were analyzed by
using theinitial rate method and Langmuir-Hinshelwood model. The
MIL-101 photocatalyst exhibited excellent catalytic recyclability
andstability and can be a potential catalyst for the treatment of
organic pollutants in aqueous solutions.
1. Introduction
Textile and paint industries and dyestuff manufacturingrelease a
considerable amount of wastewater with dyes. Thishas raised serious
environmental concerns all over the world;thus, their removal is of
interest to many scientists [1]. Dyesare difficult to treat along
with municipal waste treatmentoperations due to their complicated
chemical structures.Remazol Black B (RBB) is a popular diazo
reactive dye andused widely in textile industries [2]. Various
processes foreliminating RBB from aqueous solutions including
adsorp-tion, electrochemistry, and biosorption have been
reported.ThiThanhet al. [3] reported the efficient removal
ofRBBusingiron-containing zeolite imidazole framework-8
(Fe-ZIF-8).Fe-ZIF-8 possessed high stability. After three cycles,
the deg-radation yield was reduced slightly—95% compared to the
initial catalyst. Soloman et al. [2] reported the degradationof
hydrolyzed Remazol Black using the electrochemicalapproach.
Brazilian pine-fruit shells (Araucaria angustifolia)in natural form
are efficient adsorbents for the removal ofRBB dye from aqueous
effluents [4]. Biosorption of an azodye by growing fungi
(Aspergillus flavus) was reported inwhich the removal of chemical
oxygen demand (COD) wasfound to be 90% at 100mg·L-1 initial
concentration of dye[5]. The introduction of iron to ZIF-8
significantly enhancedthe photocatalytic degradation of RBB
Fe-ZIF-8 under visiblelight [6].
Metal-organic frameworks (MOFs) are porous mate-rials formed via
strong metal-ligand bonds between metalcations and organic linkers
[7, 8]. MOFs have many appli-cations in gas storage [9–15],
separation [16, 17], and het-erogeneous catalysis [18–21]. MIL-101
is a member of the
HindawiJournal of NanomaterialsVolume 2019, Article ID 6061275,
15 pageshttps://doi.org/10.1155/2019/6061275
http://orcid.org/0000-0003-4144-4241http://orcid.org/0000-0002-4316-4860http://orcid.org/0000-0003-3473-6377https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2019/6061275
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large family of MOFs with the largest Langmuir surfacearea
(4500m2·g-1), pore size (29–34Å), and cell volume(702.000Å). It was
first reported by Férey et al. in 2005[22], who synthesized it from
HF-Cr(NO3)3-1,4-dicarboxylicacid- (H2BDC-) H2O. Since the discovery
of large-pore MIL-101, several groups have tried to synthesize
MIL-101 for gasadsorption. However, it is hard to obtain
crystalline MIL-101 with a high BET surface of more than 3200m2·g-1
[18]because of the presence of H2BDC residue or inorganicimpurities
in the pores as well as outside the pores. In thesynthesis of
MIL-101, it is complicated to remove most ofthe nonreacted H2BDC
present both outside and within thepores of MIL-101. Yang et al.
[13] used various alkalis, suchas potassium hydroxide (KOH),
tetramethylammoniumhydroxide (TMAOH), triethylamine ((C2H5)3N),
dimethy-lamine (C2H7N), methylamine (CH5N), and ammonia(NH3), to
avoid recrystallization of H2BDC. The TMAOH-Cr(NO3)3-H2BDC-H2O
system was found to be suitablefor obtaining MIL-101 with high
surface properties. Honget al. [18] reported a separation process
in which hugeamounts of H2BDC were separated with a fritted glass
filter,and then excess dissolution was done with a hot
solvent(ethanol or N,N-dimethyl-formamide (NH4F)).
The heterogeneous photocatalysis is one of the highlyeffective
methods used for the treatment of a wide varietyof organic
pollutants owing to its ability to degrade the pol-lutants
completely. Photocatalytic degradation is usuallyconducted for
dissolved compounds in water, at mild tem-perature and pressure
conditions, using UV radiation andphotocatalytic semiconductors
without any requirement ofexpensive oxidants. The use of
metal-organic frameworks(MOFs) as photocatalysts is a new field of
application forthis material. Recently, some MOF materials such as
MOF-5 [23–25], MIL-125 [26], and MIL-53(M) (M = Fe, Al, andCr) [27]
have been applied successfully in the decolorizationof various dyes
in aqueous solutions. Owing to its excellentporosities (high
specific surface area, large pore volume,and uniform pores),
MIL-101 is a good candidate in catalysis[28] and has been found to
be a great candidate for manyother applications [16, 17, 29–32].
Several papers havereported visible-light photocatalytic activity
of modifiedMIL-101, e.g., N-K2Ti4O9/MIL-101 composite [33]
andBi25FeO40/MIL-101/PTH [34]. To the best of our knowl-edge, no
attention has ever been paid to the study of thephotocatalytic
properties of pure MIL-101 to date.
In the present paper, we focused on an Cr(NO3)3-H2BDC-H2O system
and analyzed the effect of differentconditions on the synthesis of
MIL-101 and monitoredthe hydrothermal stability of MIL-101 in
various solventsand conditions. Photocatalytic degradation of RBB
wasalso investigated.
2. Experimental
2.1. Materials. Chromium(III) nitrate nonahydrate(Cr(NO3)3·9H2O;
99%), benzene-1,4-dicarboxylic acid(C6H4(COOH)2; >98%) (denoted
as H2BDC), and hydrogenfluoride (HF; 40%) were purchased from
Merck, Germany.Remazol Black B (C26H21N5Na4O19S6, molecular weight
=
991 82) (denoted as RBB) was procured from the ThuyDuong Textile
Company (Hue city, Vietnam). The structureof RBB is shown in Scheme
1.
Ferrous ammonium sulfate (Fe(NH4)2(SO4)26H2O;>98%), potassium
dichromate (K2Cr2O7; >99%), ferroussulfate heptahydrate
(FeSO4·7H2O; >99%), silver sulfate(Ag2SO4; >99%), conc. H2SO4
(98%), and mercuric sulfate(HgSO4; >98%) were supplied from
Merck, Germany, andused to measure the COD of samples.
2.2. MIL-101 Synthesis. MIL-101 was synthesized accordingto an
earlier report with some modifications [22]. The mix-ture of
reactants including H2BDC, Cr(NO3)3, HF, andH2O was heated in a
Teflon-lined stainless steel autoclaveat 200°C for 8 h. The
resulting green solid material was fil-tered using a 0.2μmmembrane
and then extracted in ethanolwith Soxhlet equipment for 12 h to
remove residual amountof H2BDC still present in the product. The
effects of themolar ratio of chromium nitrate and water to H2BDC
onthe formation of MIL-101 were also monitored. With a fixedwater
volume of 100mL, the composition of the synthesizedgel was
calculated at the following molar ratio:
(i) For the study on the effect of the molar ratio ofCr/H2BDC,
the molar composition of a reactant mix-ture of H2BDC Cr NO3 3 ·
9H2O HF H2O =1 00 x 0 25 265, with x = 0 5, 0 75, 1 00, 1 25,1 50,
and 1 75. The samples were denoted as M-0.5,M-0.75, M-1.00, M-1.25,
M-1.50, and M-1.75
(ii) For the study on the effect of the ratio ofH2O/H2BDC, the
molar composition of a reactantmixture of H2BDC Cr NO3 3 · 9H2O HF
H2O =1 00 1 25 0 25 y, with y = 200, 265, 350, 400, 500,and 700.
The samples were denoted as M-200, M-265, M-350, M-400, M-500, and
M-700
2.3. Photocatalytic Performance. The photocatalytic degra-dation
of RBB was measured at ambient conditions usinga set of home-made
equipment. The source of UV lightwas UV-B313 30W (λ = 310 nm).
The photocatalytic experiments were performed in a1000mL beaker
containing 500mL of aqueous suspensionsof 10–50 ppm RBB and a
0.25mg catalyst. The beaker waskept at 25°C with a thermostat. The
UV lamp was focusedon the beaker at 15 cm distance. All of the
experiments wereperformed under natural pH conditions (around
7–7.5)unless specified otherwise. Then, 3mL of the mixture
waswithdrawn at certain time intervals and centrifuged toremove the
MIL-101 solid. The RBB concentration was mon-itored by means of
spectroscopy at the maximum wavelength(λ = 600 nm). The experiments
were replicated three times.
The chemical oxygen demand (COD) of the RBB solu-tion was
measured by the ASTM method [35]. The samplewas oxidized by the
boiling mixture of chromic and sulfuricacids. The sample was
refluxed in a strongly acidic solutionwith a known excess of
potassium dichromate (K2Cr2O7).After digestion, the remaining
unreduced K2Cr2O7 wastitrated with ferrous ammonium sulfate to
determine theamount of K2Cr2O7 consumed and the oxidizable
matter
2 Journal of Nanomaterials
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was calculated in terms of the oxygen equivalent. The sam-ples
were analyzed in duplicate to yield reliable data.
X-ray diffraction (XRD) was carried on a D8 ADVANCEsystem
(Bruker, Germany) (Hanoi city, Vietnam). Cu Kαradiation (λ = 1
5406Å) was the light source with appliedvoltage of 35 kV and
current of 40mA. Transmission elec-tron microscopy (TEM) was
carried out using a JEOL JEM-2100F microscope (Hanoi city,
Vietnam). Nitrogen adsorp-tion/desorption isotherm measurements
were conductedusing a Micromeritics 2020 volumetric adsorption
analyzersystem (Hanoi city, Vietnam). The samples were pretreatedby
heating under vacuum at 150°C for 3 h. The BET
(Bru-nauer–Emmett–Teller) model was used to calculate thespecific
surface area using adsorption data in the relativerange 0–0.24.
Total volume was obtained from the nitrogenvolume adsorbed at a
relative pressure of 0.99. The X-rayphotoelectron spectroscopy
(XPS) was conducted using aShimadzu Kratos AXIS Ultra DLD
spectrometer (Japan).Peak fitting was performed by CasaXPS
software. Theabsorbance of RBB, methyl orange (MO), and
methyleneblue (MB) was measured at λmax = 600 nm, 464 nm, and664
nm, respectively, using a Lambda 25 Spectrophotome-ter,
PerkinElmer, Singapore (Hue city, Vietnam).
3. Results and Discussion
3.1. Synthesis of MIL-101 and Its Stability under
DifferentConditions. The XRD patterns of the MIL-101 samples
syn-thesized with different Cr/H2BDC molar ratios of 0.50,0.75,
0.85, 1.00, 1.25, and 1.75 are shown in Figure 1(a). Fromthe
figure, it is found that the diffraction at a small angle ofabout
1.7° characterizing the mesoporous structure of MIL-101 was
obtained depending on the Cr/H2BDC ratio. At alow Cr/H2BDC ratio,
the diffraction at such a small anglewas not observed. However,
when the Cr/H2BDC ratio wasincreased to a level such as M-1.25,
M-1.5, or M-1.75, theXRD patterns were similar to the patterns of
MIL-101 asreported earlier [22], in which sharp and strong
diffractionat 2θ of around 1.7° was clearly observed. Therefore,
aCr/H2BDC ratio larger than 1.25 could provide MIL-101with high
crystallinity. The textural properties of MIL-101samples
synthesized at different ratios of Cr/H2BDC wereinvestigated by the
N2 adsorption-desorption isotherms at77K as shown in Figure 1(b).
The isothermal curves are oftype IV, with pore-filling steps at
p/p0 ≈ 0 2 and p/p0 ≈ 0 3,characteristic of the presence of two
types of narrow meso-pores [18]. The parameters characterizing the
textural prop-erties of the obtained MIL-101 samples are displayed
inTable 1. The specific surface area tends to increase with
theincrease in the molar ratio of Cr/H2BDC and reaches the
highest value at the ratio of 1.25 and then decreases when
thisratio continues to rise.
The morphologies of MIL-101 samples synthesized withdifferent
molar ratios of Cr/H2BDC were observed by TEMimages (Figure 2). It
can be seen that the particles have anoctahedron shape with
different sizes, in the range of 230-570 nm, depending on the molar
ratios of Cr/H2BDC. Thevalues of standard deviation (SD)/mean
(4%-9%) were lessthan 10% in all cases indicating that particle
size distributionswere normal. The size of particles reaches a
minimum at theCr/H2BDC ratio of 1.25 (M-1.25) (see Table 1). In
fact, theeffect of the Cr/H2BDC ratio on the MIL-101 particle
sizewas not clear as the Cr/H2BDC ratio was between 0.75and 1.25.
The particle size increased significantly as theCr/H2BDC ratio is
larger than 1.25. On the contrary, crys-tallite sizes obtained from
the Scherrer equation (using(375) diffraction) was within 34.1 and
45.8 nm and theyappeared to be less affected by the Cr/H2BDC ratio.
Theseresults implied that particles consisted of several
crystals.Based on the specific surface area and morphology, the
suit-able Cr/H2BDC molar ratio for the synthesis of MIL-101was
found to be 1.25.
Figure 3(a) displays the XRD results of MIL-101
samplessynthesized with different molar ratios of H2O/H2BDC.
Theresults show that the amount of water in the compositionof
reactants has a considerable effect on the structure ofMIL-101. All
the samples with an increase in molar ratiosof H2O/H2BDC from 200
to 700 provided the characteristicdiffractions of MIL-101. However,
the peak at 2θ of about1.7° characterizing the mesoporous structure
did not appearfor the samples having a high water content
(M-500,M-700), while the same was observed clearly for the sam-ples
having a lower water content with molar ratios ofH2O/H2BDC from 200
to 400. In addition, at a high watercontent in the reactant
mixtures, a lower peak intensity wasobserved. Therefore, the water
content in the reactant mix-tures not only affected the structure
of materials but alsoreduced their crystallinity.
The isotherms of nitrogen adsorption/desorption andtexture
properties of MIL-101 synthesized with differentmolar ratios of
H2O/H2BDC are illustrated in Figure 3(b)and Table 2. It was found
that the specific surface areaincreased steadily when the molar
ratio of H2O/H2BDCincreased and peaked at the molar ratio of 350,
but afterthat, it decreased when this ratio was increased. The
watercontent in the reactant mixtures had a significant effect
onthe particle size of MIL-101 but had less effect on
crystallitesize. It can be seen in Figure 4 and Table 2 that the
particlesize decreased slightly with the increase in water
contentwhile crystallite sizes around 26.8–31.8 nm seemed to be
N = N N = NOH
NaO3SOCH2CH2O2S
NaO3S SO3Na
NH2SO2CH2CH2OSO3Na
Scheme 1: Structure of RBB.
3Journal of Nanomaterials
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unchangeable. A large amount of water resulted in
irregularshapes and formation of the needle-shaped crystals of
tere-phthalate acid. It can be inferred that a high water
contentprobably reduced the crystal growth rates [36]. Therefore,a
perfect MIL-101 crystal could not be achieved at highwater contents
in the reactant mixture, and the most perfectMIL-101 crystal was
observed in the M350 sample.
As mentioned previously, a larger amount of nonreactedH2BDC is
present in MIL-101, resulting in a decrease in itssurface area and
pore volume. The previous studies are
focused on the purification using hot ethanol, water, or
fluo-ride–anion exchange using aqueous NH4F solutions [18, 22,32].
In the present work, we proposed the Soxhlet extractionusing
ethanol solvent to purify the H2BDC. Table 3 showsthe specific
surface area and porous volume of the presentMIL-101 material
compared to some previous studies. Thesurface area and porous
volume of MIL-101 synthesized inthis study are smaller than those
reported by Férey et al.[22] but much higher than recent studies.
The proposedpurification is time-consuming but is capable for
completely
0 5 10 15 20
(13
9 5)
(10
2 2)
(822
)(7
53)
(511
)
(311
)(2
20)
1000
(cps
)
M-1.75
M-1.50
M-0.75M-1.00M-1.25
M-0.50
Inte
nsity
(arb
.)
(111
)
2 theta (degree)
(a)
0.0 0.2 0.4 0.6 0.8 1.00
100
200
300
400
500
600
700
800
900
1000
M-0.75M-1.00M-1.25
M-1.50M-1.75
Qua
ntity
adso
rbed
(g/c
m3 ,
STP)
Relative pressure (P/P0)
(b)
Figure 1: (a) XRD patterns and (b) nitrogen
adsorption/desorption isotherms of MIL-101 synthesized using
different molar ratios ofCr/H2BDC.
4 Journal of Nanomaterials
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removing H2BDC, making the obtained MIL-101 have ahigh specific
surface area.
The results of stability testing of MIL-101 under
ambientconditions over several days are given in Figures 5(a)
and5(b). Generally, the main characteristic peaks of MIL-101
could be observed in all XRD patterns. The peak at a smallangle
(2θ is about 1.7°) was not present in the samplesexposed to ambient
conditions for 15 to 30 days without dry-ing before XRD
measurements (Figure 5(a)). It is worth not-ing that this peak was
observed clearly in the dried sample,
Table 1: Textural properties of MIL-101 samples synthesized
using different molar ratios of Cr/H2BDC.
Samples SBET (m2·g-1) SLangmuir (m
2·g-1) Vpore (cm3·g-1)
Particle size∗
M± SD(nm)
Crystallite size∗∗
(nm)
M-0.75 1582 2426 0.79 231 5 ± 20 1 34.1M-1.00 2328 3833 1.23 376
± 15 4 29.3M-1.25 2946 4776 1.53 216 ± 20 3 43.7M-1.50 2642 4354
1.41 522 ± 20 0 45.8M–1.75 2414 4057 1.28 573 ± 27 2 34.6∗Mean
value (M) of particle size counted from 100 particles. SD: standard
deviation. ∗∗Crystallite size calculated from the Scherrer equation
usingdiffraction (753).
M-0.75
500 nm
(a)
M-1.00
500 nm
(b)
M-1.25
500 nm
(c)
M-1.50
500 nm
(d)
500 nm
M-1.75
(e)
Figure 2: TEM images of MIL-101 synthesized using different
molar ratios of Cr/H2BDC.
5Journal of Nanomaterials
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although it was exposed to ambient conditions up to a
year(Figure 5(b)). The peak at a small angle characteristic of
amesoporous structure seems to be unstable under moistenedair. This
explains why this peak which diffracted at 1.7° hasbeen reported in
some studies [22], while not in others [13,15, 18]. The reason is
not clear, but it could be possible thatwater in the moistened air
blocks some of the pores resultingin the disappearance of the
characteristic diffraction of themesoporous structure at around
1.7°. Therefore, this peakwas observed after the drying process to
remove water vaporfrom the material structure.
The stability of MIL-101 material in water at room tem-perature
is illustrated in Figure 5(c). The results indicate thatthe
characteristic diffractions of MIL-101 were obtained in
allpatterns. Notably, the characteristic diffraction of the
meso-porous structure at around 1.7° still remained stable afterthe
sample had been soaked in water for several days.
The stability of MIL-101 material in several solvents atboiling
temperature for 8 h is shown in Figure 5(d). Theresults of XRD
indicate that MIL-101 material was still stableafter being soaked
in boiling water continuously for 8 h,
which is in good agreement with that reported by Honget al.
[18]. Moreover, the characteristic diffraction of MIL-101 still
appeared with high intensity suggesting that itsstructure did not
collapse after soaking in ethanol and ben-zene at boiling
temperature for 8 h. In contrast, the MOFsthat are used mostly,
such as MOF-177 andMOF-5, have rel-atively high thermal and
chemical stabilities; they are knownto be unstable and to easily
decompose in the presence ofmoisture [38, 39]. Therefore, MIL-101
is rather stable in bothpolar and nonpolar solvents at high
temperatures, whichmakes MIL-101 an attractive candidate for
various applica-tions and catalysts.
Lin et al. [40] used X-ray absorption near edge structure(XANES)
spectroscopy for characterizing the oxidation stateof Cr in the
MIL-101 catalyst and found that the chromiumatom is in the divalent
(Cr(II)) state in the MIL-101 crystalsalthough the source of
initial Cr is Cr(III) in Cr(NO3)3. Inthe present paper, the surface
composition of the MIL-101sample was analyzed by XPS (Figure 6(a))
and the spectracorresponding to C1s and Cr2p were collected. The
bindingenergy values of 587 eV for Cr2p1/2 and 576 eV for
0 5 10 15 20(1
3 9
5)
(10
2 2)(753
)(8
22)
(511
)
(311
)(2
20)
M-700M-500M-400
M-265
M-350
M-200
1000
(cps
)In
tens
ity (a
rb.)
2 theta (degree)
(111
)
(a)
0.0 0.2 0.4 0.6 0.8 1.00
100200300400500600700800900
100011001200
M-350M-200M-265
M-400M-700
Qua
ntity
adso
rbed
(g/c
m3 ,
STP)
Relative pressure (P/P0)
(b)
Figure 3: (a) XRD patterns and (b) isotherms of nitrogen
adsorption/desorption of MIL-101 synthesized using different molar
ratios ofH2O/H2BDC.
Table 2: Textural properties of MIL-101 samples synthesized
using different molar ratios of H2O/H2BDC.
Samples SBET (m2·g-1) SLangmuir (m
2·g-1) Vpore (cm3·g-1) Particle size
∗
M± SD (nm) Crystallite size∗∗ (nm)
M-200 1618 2570 0.87 530 5 ± 78 2 26.8M-265 2946 4776 1.53 520 3
± 53 4 30.4M-350 3360 5059 1.44 490 3 ± 19 4 31.8M-400 2274 3664
1.25 250 ± 35 31.4M-700 1708 2701 0.93 137 ± 25 30.0∗The mean value
(M) of 50 particles counted from TEM images. SD: standard
deviation. ∗∗Crystallite size calculated from the Scherrer equation
usingdiffraction (753).
6 Journal of Nanomaterials
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Cr2p3/2 (Figure 6(b)) are typically assigned to Cr3+ [41].
Itcould be inferred that the oxidation state Cr(III) of the
chro-mium in MIL-101 does not change during its synthesis.
Figure 7(a) shows the DR-UV-Vis spectrum of MIL-101.There are
three absorption peaks at 275, 425, and 610nm.The absorption band
at the UV region could be contributedby the electron transfer n→ π∗
in terephthalic acid. Theabsorption bands in the visible region
should be related tothe electron transfer in orbital 3d. The energy
gaps basedon Tauc’s plot were found to be 1.75, 2.27, and 3.74
eV(Figure 7(b)). Since the oxidation state of chromium inMIL-101 is
3+, it was believed that here the electron shiftoccurs in the 3d3
orbital of Cr3+ under the action of the tere-phthalate ligand
field.
In order to analyze this electron transfer, the Tanabe-Sugano d3
diagram was used (Figure 8) and, according towhich, spin-allowed
transition was as follows:
4A2g → 4T2g4A2g → 4T1g4A2g → 4T1g P
1
In Tauc’s plot (Figure 7), three energy levels could be seento
be excited corresponding to the wave number: ν1 =14104 37 (cm-1),
ν2 = 18281 54 (cm
-1), and ν3 = 30120 48(cm-1). Since the ratio ν2/ν1 = 18282
54/14104 37 was foundas 1.3, this ratio corresponded to Δo/B = 36
based on theTanabe-Sugano diagram of the d3 system (Δ is the
ligand
field splitting parameter). Considering Δo/B = 36, the valueof
E/B for the spin-allowed transition was determined to beν1/B = 36,
ν2/B = 46, and ν3/B = 76.
Since ν1 was 14104.37 cm-1, the value of B calculated
from the first spin-allowed transition was 391.8 cm-1 and Δwas
calculated as 14104.37 cm-1 from the ratio Δ/B = 36.
It is worth noting that the Racah B parameter in MIL-101 is
391.8 cm-1 while that in free Cr3+ is 1030 cm-1 [42].Thus, a
reduction of about 62% in the Racah B parameterof MIL-101 compared
to Cr2O3 indicates a strong influ-ence of the terephthalate ligand.
Figure 8 presents threespin-enabled electrons in the Tanabe-Sugano
diagram ofthe d3 system corresponding to the three excited
energylevels as follows:
(i) 4A2g→ 4T2g with energy transfer of 1.75 eV corre-sponds to a
wavelength of 709nm
(ii) 4A2g→ 4T1g with energy transfer of 2.27 eV corre-sponds to
a wavelength of 547nm
(iii) 4A2g→ 4T1g (P) with energy transfer 3.74 eV corre-sponds
to a wavelength of 332nm
MIL-101 is constituted from the trimer units (Cr3O16),which are
made up of CrO6 clusters, wherein a central chro-mium atom is
surrounded by 6 oxygen atoms [18, 22]. Assuggested by Bordiga et
al. [43] for MOF-5, it is supposedthat Cr3O16 clusters in MIL-101,
which behave as quantumdots surrounded by six terephthalate
ligands, could act aslight-absorbing antennae (hν) then
transferring it to Cr3O16
500 nm
M-200
(a)
500 nm
M-265
(b)
500 nm
M-350
(c)
500 nm
M-400
(d)
500 nm
M-500
(e)
M-700
500 nm
(f)
Figure 4: TEM images of MIL-101 synthesized using different
molar ratios of H2O/H2BDC.
7Journal of Nanomaterials
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Table 3: Comparison of the porosity of the present MIL-101 and
the results published.
SBET (m2·g-1) SLangmuir (m
2·g-1) V (cm3·g-1) Method/purification References3360 5059 1.4
Hydrothermal method/Soxhlet extraction using ethanol solvent The
present work
4100 5900 2.0 Hydrothermal method/ethanol and DMF [22]
4230 — 2.2Hydrothermal method/double filtration with glass and
paper filters,
water, ethanol, and NH4F[18]
2345 3674 1.3 Microwave-assisted hydrothermal method/ethanol
[32]
3054 4443 2. Microwave-assisted hydrothermal method/ethanol and
DMF [37]
3197 4546 1.7 Solvothermal process/TMAOH ((CH3)4NOH) [13]
2220 — 1.1 Hydrothermal method/ethanol and DMF [20]
2674 — 1.4 Hydrothermal method/ethanol and DMF [9]
3360 4792 1.8 Microwave-assisted hydrothermal method/water,
ethanol, and NH4F [15]
DMF: N,N-dimethylformamide.
0 2 4 6 8 10 12 14 16 18 20
Exposed to ambient atmosphere for 1 month
Exposed to ambient atmosphere for 0.5 month
MIL-101
500
(cps
)In
tens
ity (a
rb.)
2 theta (degree)
(a)
2 theta (degree)0 5 10 15 20
Exposed to ambient atmosphere for 12 months
Exposed to ambient atmosphere for 8 months
Exposed to ambient atmosphere for 5 months
Exposed to ambient atmosphere for 4 months
Exposed to ambient atmosphere for 3 months
Exposed to ambient atmosphere for 2 months
500
(cps
)In
tens
ity (a
rb.)
(b)
0 5 10 15 20
Exposed to water for 2 days
Exposed to water for 4 days
Exposed to water for 14 days
MIL-101
500
(cps
)In
tens
ity (a
rb.)
2 theta (degree)
(c)
0 5 10 15 202 theta (degree)
Benzene
Ethanol
MIL-101
500
(cps
)In
tens
ity (a
rb.)
H2O
(d)
Figure 5: XRD patterns of MIL-101 exposed to ambient atmosphere
for several months: (a) MIL-101 sample tested without drying;
(b)MIL-101 sample dried at 100°C for 12 h before testing; (c)
MIL-101 exposed to water for several days; (d) MIL-101 exposed to
severalsolvents at boiling temperature.
8 Journal of Nanomaterials
-
1200
2
4
6
Inte
nsity
8
10
× 104
900 600Binding energy (eV)
300
Cr2p C1s
0
(a)
Inte
nsity
× 102
Cr2p1/2
Cr2p3/2
Fitting line Cr(III)
Experiment line
Background
Binding energy (eV)592
14
16
18
20
22
588 584 580 576 572 568
(b)
Figure 6: XPS spectra for the MIL-101 (a) and the binding energy
of Cr2p (b).
200 300 400 500 600 700 800
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Abs
orpt
ion
Wavelength (nm)
(a)
1 2 3 4 5 6 70
10
20
30
(𝛼.E
)2(e
V/c
m−1
)2)
40
50
1.75 eV 2.27 eV3.74 eV
E (eV)
(b)
Figure 7: (a) DR-UV-Vis spectrum; (b) Tauc’s plot of
MIL-101.
0 5 10 15 20 251
2
3
4
3.74 eV
2.27 eV
1.75 eV
(𝛼.E)2(eV.cm–1)2
00
10
20
30
402F
2G4P
4F 4A2g
2Eg
2T1g
2T2g
2A2g
4T2g
4T1g
4T1g
2A1g
50
E/B
60
70
80
10 20 30 36 40Δ0/B
50
Figure 8: The electron shift corresponding to the three excited
energy levels in MIL-101.
9Journal of Nanomaterials
-
clusters to irradiate the photons (hν′) as can be observed
inFigure 9. The photocatalysis of MIL-101 was considered tobe
achieved by the electron transfer from photoexcitedorganic ligands
to metallic clusters in MIL-101, which wastermed as a
ligand-to-cluster charge transfer [23, 25].
3.2. Photocatalytic Activity of MIL-101. Adsorption is one ofthe
important aspects of photocatalysis. Hence, in mostcases, the dark
adsorption is often conducted to obtain anadsorption-desorption
equilibrium before UV is irradiated.In the present case, that
procedure could not be carriedout because the dye molecules were
adsorbed quickly onthe MIL-101 surface and prevented the UV light
from stim-ulating the MIL-101 catalyst, so that the UV light was
irra-diated at the same time as the catalyst was introduced asshown
in Figure 10. Figure 10(a) shows the decolorizationkinetics for RBB
dye under different conditions. It wasfound that the color of the
RBB solution with MIL-101 cat-alyst but without UV irradiation was
decolorized around43% within only 15min and then stayed constant
due tosaturated adsorption while the RBB color was
removedcompletely under UV irradiation/MIL-101 catalysis
after45min. These experiments demonstrated that the decolori-zation
of the solution was due to the photocatalytic effectrather than
adsorption.
The kinetics of decolorization of RBB on MIL-101 addedwith 0.01
g Cr(NO3)3 or Cr2O3 are shown in Figure 10(b). Itwas found that the
Cr3+ ion or Cr2O3 did not affect decolor-ization. It could be
inferred that the Cr3+ ion or Cr2O3 doesnot exhibit photocatalytic
activity in this condition. In addi-tion, the decolorization of RBB
was not observed if UV wasirradiated in the absence of MIL-101
suggesting that RBBwas stable and did not undergo photolysis. A
leaching exper-iment was also conducted in which the MIL-101
catalyst wasfiltered by centrifugation after 5min of irradiation.
Thedecolorization of dye was stopped despite the fact that UVlight
irradiation was still maintained. This indicates thatthere is no
leaching of the active species, into the reactionsolution, from the
homogeneous catalyst. The above experi-mental results confirmed
that MIL-101 was a heterogeneouscatalyst in the degradation
reaction of RBB.
The influence of the initial concentration of RBB on
thephotocatalytic decolorization rate in the presence of MIL-101 is
shown in Figure 11. The results exhibited that whenthe dye
concentration increased in the range of 10 ppm to50 ppm, an
increase in the decolorization rate was observed.
The generalized rate equation for decolorization of dyecan be
written as
r = − dCdt
= k · Cn, 2
where C is the concentration of dye at time t (the
reactiontime), k is the kinetic rate constant, n is the order of
the reac-tion, and r is the reaction rate.
In this paper, the initial rate method was used to deter-mine k
and n [44, 45].
The instantaneous reaction rate was calculated from thefollowing
equation:
rin = −dCdt
3
Integrating equation (3) for the boundary conditionst→ 0, then
C→ C0 gives
Ct = −rin ⋅ t + C0, 4
where C0 and Ct are the initial concentration and the
con-centration at time t, respectively. The instantaneous rate(rin)
is determined from the plot of concentration versustime at time t.
The initial rate (r0) of a reaction is the instan-taneous rate at
the start of the reaction (when t = 0). Theinitial rate is equal to
the negative slope of the curve of reac-tant concentration versus
time at t = 0. From the slopes ofthe plots of Ct against t at C0 (t
= 0), the values of r0 corre-sponding to each initial concentration
C0 was obtained asshown in Figure 12(a).
On the other hand, the initial rate for a reaction can bewritten
as
r0 = ki · Cn0 , 5
where ki is the overall observed rate constant for the
reactionand n is the order of the reaction with respect to the
concen-tration. The linearization of equation (5) by taking
naturallogarithms on both sizes yields
ln r0 = ln ki + n · ln C0 6
Therefore, the plot of the ln r0 against ln C0 gives astraight
line with a slope corresponding to n and the inter-cept on the
ordinate gives ln ki (Figure 12(a)). From the plotof ln rAo against
ln CAo, the slope, n = 0 604, and k = 1 156
h𝜐
h𝜐
h𝜐
h𝜐′
h𝜐′
h𝜐′
Figure 9: The Cr3O16 cluster of MIL-101 (Cr (green rods), O(red
rods), and benzene ring (blue ring)).
10 Journal of Nanomaterials
-
were calculated, and the plot has an excellent
correlationcoefficient (r2 = 0 998, p ≤ 0 001). The reaction order
ofphotocatalytic degradation is unity in some cases [27, 46].In the
present paper, the value of n less than unity could
be due to the contribution of both adsorption and
photocat-alytic reaction.
The Langmuir-Hinshelwood (L-H) equation is widelyused in
studying the kinetics of photocatalytic reaction
0 10 20 30 40 500.0
0.2
0.4
0.6
0.8
1.0
t (min)
Catalyst with UV irradiationCatalyst without UV irradiationUV
without catalyst
C/C
0
(a)
0 10 20 30 40 500.0
0.2
0.4
0.6
0.8
1.0
t (min)
MIL-101 (Cr) with UV irradiationRemoving MIL-101 (Cr) after 5
minutesCr2O3Cr (NO3)3
C/C
0(b)
Figure 10: (a) The time dependence on decolorization efficiency
of the RBB dye with or without MIL-101 catalyst; (b) leaching
experiments.
0 10 20 30 40 50 60 70 800
5
10
15
20
25
30
35
40
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50
5
10
15
20
25
30
35
40
t (min)
t (min)
Ct (
mg
L–1 )
Ct (
mg
L–1 )
10 mg L–1
20 mg L–1
30 mg L–1
40 mg L–1
50 mg L–1
Figure 11: The plots of Ct against t (time) and tangent lines at
C0; the inset presents extrapolated tangent lines.
11Journal of Nanomaterials
-
[46, 47]. In this model, the reaction rate depends on the
per-centage of the surface coverage, θ, by the following
equation:
r0 = kT · θ = −dCdt
=kT · KLH · C01 + KLH · C0
, 7
where KLH is the Langmuir-Hinshelwood adsorption equi-librium
constant (L·mg-1) and kT is the reaction rate con-stant
(mg·L-1·min-1). Equation (7) can be written in thelinear form
as
1r0
=1 + KLH · C0kT · KLH · C0
=1
kT · KLH · C0+
1kT
8
The plot of 1/r0 against 1/C0 (Figure 12(b)) gives astraight
line with a good correlation (r2 = 0 987, p = 0 001); the values of
kT and KLH were 17.857mg·L
-1·min-1 and0.035 L·mg-1, respectively. Akpan and Hameed [46]
com-pared the adsorption and reaction rate relying on the ratioof
kT KLH. However, this comparison seems to be unclearbecause the
units of kT and KLH are not similar.
The complete degradation of dyes with a cost-effectiveprocess is
important for industrial applications. UV-Visspectra are shown in
Figure 13(a). The absorbance band at310nm was contributed by the π→
π∗ transition of the dou-ble bond in the aromatic ring. The
absorbance band at600nm was assigned to n→ π∗ due to the double
bond con-jugation of N=N and C=C in the aromatic ring. The
absorp-tion peak decreased significantly with an increase in
theirradiation time and disappeared after 45min of irradiation.The
chemical oxygen demand (COD) test is shown inFigure 13(b). The COD
decreased from an initial value of86.4mg·L-1 to around 10.0mg·L-1
after 80min. The resultsindicate that RBB molecules were degraded
into fragmentsand, subsequently, were completed with minerals.
The photochemical degradation mechanism of RBB dyeon MIL-101 can
be interpreted by the semiconductor theory[25]. As proved earlier,
MIL-101 was photoexcited leading tothe electron transitions in the
3d3 orbital, followed by the for-mation of an electron (e-) and
hole (h+) pair on the surface ofthe catalyst. The high oxidation
potential of the hole (h+) inthe catalyst either permitted the
direct oxidation of the dyeor reacted with water molecules or
hydroxyl ions (OH-) togenerate hydroxyl radicals (⋅OH). These
hydroxyl radicalsoxidized the surface adsorbed organic molecules.
On theother hand, the photogenerated electrons (e-) reduced thedye
or reacted with the O2 adsorbed on the MIL-101 surfaceor dissolved
in water forming a radical anion (⋅O2
-). Thisstrong oxidation could degrade RBB. According to this,
thephotochemical degradation reactions of RBB on MIL-101can be
expressed as follows:
MIL‐101 + hv MIL‐101 e− + h+
h+ + dye→ RBB⋅ → degradation productsh+ + H2O→H++⋅OHh+ + OH− →
⋅OH
⋅OH + RBB⋅ → degradation productse− + O2 → ⋅O2−
⋅O2− + RBB→ degradation products9
The reusability of the catalyst is an important concern inthe
application of heterogeneous catalysis. After the experi-ment, the
MIL-101 material was collected by centrifugationand washed with
water and ethanol for three times toremove RBB completely and dried
at 120°C for 15 h andthen reused. The photocatalytic degradation
efficiency ofMIL-101 after four cycles of usage was decreased
slightly
2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.01.4
1.6
1.8
2.0
2.2
2.4
2.6
y = 0.604⁎x + 0.145r2 = 0.998, p = 0.001
ln (C0)
ln(r
0)
(a)
0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
y = 1.596⁎x + 0.056r2 = 0.987, p = 0.001
1/r 0
1/C0
(b)
Figure 12: (a) Initial rate plot for RBB to determine the
overall rate order; (b) a plot of the Langmuir-Hinshelwood
model.
12 Journal of Nanomaterials
-
(only 5%) (Figure 14(a)). The XRD patterns of the
recycledMIL-101 catalysts were unchanged (Figure 14(b)). It can
beconcluded that theMIL-101 photocatalyst exhibited
excellentcatalytic recyclability and stability in the tested
conditions.It can be inferred that MIL-101 can be a potential
catalystfor the treatment of organic pollutants in aqueous
solutions.
4. Conclusions
The suitable molar ratios of Cr/H2BDC and H2O/H2BDC forthe
synthesis of MIL-101 with a large surface area and
highcrystallinity were 1.25 and 350, respectively. High water
con-tent in the synthesized gel significantly reduced particle
sizeand crystallinity. MIL-101 can be exposed to ambient
condi-tions for several months. The characteristic XRD
diffraction
pattern at 1.7° can be observed in the samples stored in
dryconditions, whereas this diffraction disappeared when thesamples
were stored in moistened conditions. MIL-101 is sta-ble in water,
benzene, and toluene even at boiling point forseveral hours.
MIL-101 exhibited excellent photodegradationof RBB in the UV
region. The kinetics of the photocatalyticdegradation reaction was
fitted well with the Langmuir-Hinshelwood (L-H) equation. The
initial rate method yieldedthe order of the reaction and the
initial reaction rate constantto be 0.604 and 1.156
[(mg·L-1)0.396·min-1], respectively.
Data Availability
The data used to support the findings of this study areavailable
from the corresponding author upon request.
300 400 500 600 700 800−0.1
0.00.10.20.30.40.50.60.70.80.91.0
Abs
orba
nce
Wavelength (nm)
Initial 5 min10 min
25 min45 min720 min
(a)
0 100 200 300 400 500 600 700
10
20
30
40
50
60
70
80
90
Time (min)
COD
(mg.
L–1 )
(b)
Figure 13: The UV-Vis spectrum (a) and the chemical oxygen
demand (COD) test (b) of the RBB dye and RBB degradation over
MIL-101(Cr) under UV irradiation.
0
20
40
60
80
100
4threcycle
3rdrecycle
2ndrecycle
1strecycle
Deg
rada
tion
effici
ency
(%)
Rawmaterial
(a)
0 5 10 15 20
(10
2 2)
(753
)(8
22)
(511
)
(311
)(2
20)
4th recycle
3rd recycle
2nd recycle
1st recycle
Inte
nsity
(abr
.)
2 theta (degree)
1000
(cps
)
Raw material
(111
)
(b)
Figure 14: (a) The RBB degradation efficiency of the MIL-101
photocatalyst for the fourth cycle; (b) XRD patterns of MIL-101 and
usedMIL-101 (V = 450mL; C0 = 90mg · L−1, UV-irradiation time:
45min).
13Journal of Nanomaterials
-
Conflicts of Interest
The authors declare that they have no conflict of interest.
Acknowledgments
This research was sponsored by Hue University underDecision No.
1208/QĐ-DHH.
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